Globally Changing Nutrient Loads and Harmful Algal Blooms: Recent Advances, New Paradigms, and Continuing Challenges

It is now well recognized that there are more harmful algal blooms (HABs), more often, in new and different places, often lasting longer, and with a range of toxicities, and that many of these blooms are related to nutrient pollution. Nutrient loads are increasing globally, but they are changing regionally in proportion and in the dominant form of nutrient. The fact that nutrient loads have generally increased is, in itself, insufficient for the promotion of HABs. The success of HABs lies at the intersection of the physiological adaptations of the harmful algal species, environmental conditions, and interactions with co-occurring organisms that alter abiotic conditions and/or aggregate or disperse cells, in turn promoting or inhibiting their growth. It is a change in the supply of the right nutrients at the right time that helps to create conditions conducive to specific HABs. Many dinoflagellate and cyanobacterial HABs appear to have adaptations that allow them to exploit environments-and potentially even become more toxic-where nutrients are not in balanced (Redfield) proportions. HABs are also changing in complex ways due to changes in climate and many other changes that affect the timing, amount, or proportions of nutrients. There is much work to be done to understand the physiological ecology of HABs and other co-occurring species. Climate change and altered CO 2 levels, and their implications for altered productivity of the global ocean, should motivate both new models and new experimental investigations that support them. Based on current knowledge, the management implications are clear: the most effective actions to reduce HABs and their impacts will be continued efforts to reduce nutrients entering our waters.